1. Field of the Invention
The invention is concerned with security elements for security articles such as banknotes, cheques and the like.
2. The Prior Art
Holograms are widely used as security features in credit cards as they cannot be reproduced by photographic or photocopying techniques. To enhance the security of holograms and to prevent contact copies being made, a technique was developed for making holograms by a process of demetalization. Demetalized holograms and patches are used in passports and ID cards to protect photographs and data. The image beneath the hologram is only visible when the document is tilted. Such holograms therefore provide very useful surface security features and can be more easily used in applications where the substrate to which they are applied is flexible or foldable such as in banknotes.
It is also widely known to use in banknotes security elements in the form of strips or threads which are made from a transparent film provided with a continuous reflective metal layer, vacuum deposited aluminium on polyester film being the commonest example. Banknotes made using such security devices have been in general circulation in many countries for many years. When such security elements are embedded in security paper and the paper is subsequently printed to provide the security document, e.g. a banknote, the thread cannot readily discerned in reflected light but is immediately apparent as a dark image when the document is viewed in transmitted light. Such threads are extremely effective against counterfeiting by printing or photocopying, since the optically variable effect which is provided cannot be accurately simulated, for example by printing a line on the paper.
The composition, size and positioning of security threads currently in use vary depending on the desired security of the document. Typical security threads are composed of a polymeric film such as polyester, which may be metallised or coloured and may include microprinted lettering denoting a title or message. The lettering can be produced by printing onto the substrate or by de-metallising a metallic layer on the substrate. One example of a microprinted thread can be found in GB-A-1095286. The threads can be slit to produce either registered lettering with respect to the edge of the thread, or unregistered lettering designed so that the message always appears irrespective of thread slitting. The widths of threads typically used vary from 0.5 mm to 2 mm and may have thicknesses typically ranging from 12 micrometers up to 50 micrometers.
The positioning of the thread within the document may be strictly controlled to agreed criteria and may be concurrent with additional security features, such as a watermark. Security threads may be fully embedded within security paper in such a way that paper fibres cover both sides of the thread, making it considerably less visible in reflective light, but clearly visible in transmitted light.
In recent times, however, in order to enhance security documents against modern counterfeiting techniques making use of sophisticated colour separation, printing and colour photocopy technology, it has become common to use a security thread comprising a thin layer of aluminium on a plastic support which is exposed on one side of the sheet at intervals along the length of the thread, the region of exposure being referred to as a window. GB-A-1552853 and GB-A-1604463 disclose banknotes containing such windows. Paper for use in producing such banknotes can be made using the method disclosed in EP-A-0059056. The dimensions of the windows typically used are from 3 mm to 14 mm lengthways, with bridges ranging from 4 mm to 30 mm there between. Again, the positioning of the windows may be controlled to allow registration of the window with respect of the document and other security features such as watermarks.
This latter development has resulted in enhanced security, and windowed paper has been used for banknotes in many countries. A banknote of this type provides added security against counterfeiters as, when viewed in transmitted light, the strip is seen as a dark line and when viewed in reflected light on the appropriate side, the bright shining aluminium portions which are exposed at the windows are readily visible. However, there is a need for even greater security by the use of more sophisticated security devices in order to render the task of a would-be counterfeiter more difficult as the reflected light appearance of the exposed aluminium portions of a security device can be simulated to a degree by modern materials and techniques, for example, by the use of hot foil stamping.
Other threads in use are coated with protective lacquers and these may contain, for example, fluorescent inks which are only visible on illumination with ultra violet light of a specific wavelength, e.g. 366 nm.
It is also possible to make threads that are machine readable by incorporating into or onto the substrate, substances which can be identified by a detector, such as magnetic material.
Other known threads can be a single layer or multiple layers bonded or laminated together, and in some instances each layer may be coated with metal such as aluminium, stainless steel, tin and/or metal oxide such as tin oxide. The coating may be protected with a lacquer which may also contain coloured or invisible fluorescent pigment, for example, rare earth chelates, such as europium acetylacetonate and the like.
Another prior art solution to enhancing the security of a security article such as banknotes is described in DE-B-1228972. This utilises a thermochromic compound applied in the form of a picture or other marking to paper such as a banknote such that when it is warmed the previously invisible picture or marking is revealed. This effect is reversible such that the picture or marking disappears when the banknote is cooled.
EP-A-0243285 also describes security paper which has a thermochromic coating. The paper is authenticated by applying heat which results in a change in the colour of the paper itself. EP-A-0400220 describes a laminated article having two layers, of which one is transparent and between which is a layer carrying or containing thermochromic liquid crystals in the form of a pattern which can be made visible or obtains a change in colour on a heat change.
In EP-A-0608078 there is described an improved security thread for use in security articles which has a thermochromic coating on one or both sides of the thread. The coating comprises a material selected from pigments and dyestuffs and the coating overlays indicia on the substrate such that when the thermochromic material is heated to its activation temperature, the coloured coating turns clear to reveal the indicia (or vice versa).
The present invention is concerned with providing a further improvement to this type of security thread by increasing the number of security features utilised.
The invention therefore provides a security element for use in security articles, said element comprising a first layer, to one side of which is applied an optically variable device, and on the other side of said first layer is applied indicia, and thermochromic coating applied over the indicia, wherein the thermochromic coating comprises a thermochromic material which is coloured when a temperature of said thermochromic coating is below an activation temperature, which colour obscures the indicia and enhances the visibility of the optically variable device, and wherein the thermochromic material becomes colourless at or above said activation temperature to thereby reveal the indicia, and to render the optically variable device substantially invisible in reflected light.
The present invention also provides a security element for use in security articles said element comprising a first transparent layer, to one side of which is applied an optically variable device and on the other side of said first layer are applied indicia, and a thermochromic coating applied over the indicia, wherein the thermochromic coating comprises a thermochromic material which is colourless when the temperature of said thermochromic coating is below an activation temperature, which will thereby reveal the indicia and render the optically variable device substantially invisible in reflective light, and wherein the thermochromic material becomes coloured above said activation temperature, which colour substantially obscured the indicia and enhances the visibility of the optically variable device.
The invention will now be described, by way of example only, with reference to the accompanying drawings.